Yes, this is what the iSub was. However, it doesn't work with Tiger Apple apparently are working on it, but the last update bought no changes.
Other than that, there are none that I am aware of, and there's no way of using the headphones connection at the same time as the built in speakers to drive a subwoofer as far as I'm aware.
TBH, the built in speakers are great, but you'd be far better off just getting a discreet set of computer speakers...
I have the Harman Kardon iSUB Subwoofer (before I upgraded to the one with soundsticks), and have been pretty pleased with the sound quality.
... I'm also impressed with the sound that my JBL onstage II produces and I thought that the JBL 'creature II' subwoofer came standalone as well. But the website it seems JBL does not sell stand-alone subs (maybe I'm looking in the wrong section)? The apple store has no stand alone subwoofers either... it seems speakers + subwoofer packages are the norm.
As an Amazon Associate, MacRumors earns a commission from qualifying purchases made through links in this post.
I've got the 24" and it seems like a waste to have such awesome speakers and not use them. I plug my surround sound system in when I watch movies, then unplug them after.